The effect of changes in temperature on the operation of some circuits can be compensated for by including one or more resistors therein having an ohmic value that changes by an appropriate amount for each change in temperature of one degree. Such resistors are usually made of material such as gold or platinum that has a high temperature coefficient TC, i.e., a high fractional change in resistance for each change of one degree. The actual change per degree in resistance of such a resistor from the value RT it has at a given temperature is equal to the product of RT and the temperature coefficient TC of the material from which it is made.
Some electrical equipment is comprised of a device requiring temperature compensation that is coupled to a printed circuit containing the temperature compensating resistor. In most cases, each device will require a different temperature compensating resistor. If discrete resistors are to be used, each device is tested to determine the ohmic change with temperature that the temperature compensating resistor must provide and such a resistor is selected and connected to the printed circuit. This procedure has the disadvantage of requiring a large number of resistors with different temperature characteristics to be kept on hand. Furthermore, it is often difficult or expensive to connect the resistors to the printed circuit. It would, therefore be advantageous to make the resistor an integral part of the circuit.
It would be possible to provide a resistor that is an integral part of the printed circuit that is made of material having a high temperature coefficient and trim it with a laser or other means until some measured voltage or current in the circuit device indicated that the correct value had been attained, but the heat usually introduced by the trimming would change the resistance of the resistor appreciably so that it would be necessary to let it cool before the circuit could give an accurate indication. In order to avoid trimming too much, it would be necessary to trim a little bit at a time in each of a series of steps and wait for the resistor to cool after each one.